In prior arts, a slurry is prepared by mixing a YAG phosphor and a binder such as a silicone, and the slurry is applied to a blue light LED by a dispenser or the like to change light emission characteristics.
Patent Literature 1 owned by the applicant of this patent application discloses a technique of applying a phosphor to a surface and a side of an LED to which it is not possible to apply a phosphor by a dispenser to form a thin film while controlling the quantity of the phosphor applied.
Patent Literature 2 discloses a method for manufacturing a white light emission diode including a blue light emission diode and phosphor layers for conversion into yellow light and red light, which phosphor layers are produced by aerosol-depositing fluorescent particles on the blue light emission diode.
Patent Literature 3 discloses a method invented by the same inventor as that of the present invention. Patent Literature 3 discloses a method called screen spray, in which openings of a rotary screen are filled with a powder or granular material by a positive displacement system, and then the powder or granular material is extruded out to the side opposite to the filling side by compressed gas or the like for application.
In cases where a slurry is applied using ordinary dispensing or spraying, not only a slurry layer formed on the surface of an LED but also a slurry layer formed on the side of the LED is not uniform, and the color temperature is not uniform among the portions of the chip, leading to critical defects such as yellowing or bluing.
As a countermeasure to this problem, filling of slurry is performed using a dam around an LED chip or using a reflector. This makes the process complex and requires to make the quantity of binder such as silicone resin larger than the quantity of phosphor in order to improve fluidity of slurry, which makes the film thickness larger than required, leading to deteriorated performance due to light loss.
By the method disclosed in Patent Literature 1, the proportion of phosphor can be made larger, and a thin layer can be formed on an LED directly. Therefore, high quality LEDs with small light loss can be produced by this method. Moreover, a uniform thin layer can be formed not only on the top surface of an LED but also on the side surface. For this reason, this method has been receiving attention as a method that can manufacture high-added-value LED packages.
Generally, the area ratio of a substrate and an LED chip on the substrate is between ¼ and 1/30, and phosphor is applied on the entirety of the substrate. Therefore, the use efficiency of phosphor is very low.
Moreover, in cases where molding of a lens using a silicone resin or the like is performed afterward, a mask is used so as not to coat the portions around the LED to prevent deterioration in adhesion of the silicone resin for molding and a phosphor rich layer around the LED. The phosphor adhering to the mask contains reactive curing silicone, and it is difficult to recycle it.
In aerosol deposition disclosed in, for example, Patent Literature 2, a deposited film can be formed on an object by setting the object in a chamber kept at a high degree of vacuum of e.g. 0.4 to 2 Torr, fluidizing powder or granular material by gas, and transferring micro particles of ceramics or the like having diameters of 0.08 to 2 micrometers by energy of differential pressure higher than 50 kPa to cause them to impinge on the object at a speed higher than 150 m/sec. However, since it employs fluidization, there still remains the problem of film thickness distribution in the deposited film per microscopic unit area, because even on the aforementioned micron order, the aforementioned smaller size particles and larger size particles show different flow behaviors, even if a pulverizer or classifier is used. Moreover, if deposition is performed with phosphor having an average particle diameter of 15 micrometers, there arises a problem that a portion of an LED such as a wire is broken by impact energy.
The average particle diameter of yellow phosphor for LEDs is generally between 7 and 30 microns. There is a variation in particle size as a matter of course. For example, in the case where the average particle diameter is 15 microns, the particle size distribution spreads between several microns and 60 microns. Therefore, the particle concentration is uneven in the fluidized state. Since the weight of phosphor per square centimeter is very small (e.g. 5 mg or so in the case of the normal white color temperature), and the particles move in several milliseconds when transferred, the variation per unit time is large. If the phosphor is fluidized in a fluidization chamber with an increased quantity of gas, namely with a decreased concentration of the phosphor, it is difficult to stabilize the application quantity with the lapse of time, because heavy particles tend to sink and light particles tend to float.